Yes, that's exactly right. Since the high and low beams share a common ground, if that ground isn't there, the voltage will back up through the other filament, go to the opposite light, and seek a ground there. That puts two filaments in series, and dramatically reduces the brightness. In the case of rear lights with one bad ground, both tail lights will appear to be close to normal brightness, but one will go out when the brakes are applied.
Jerry
